In oil exploration and geophysical surveys, it is useful to provide measuring systems which can detect locations of boundaries between different formations. Examples of applications for such systems include reservoir characterization, geo-steering in directional drilling, salt dome mapping for engineering of gas storage caverns and delineation of hydrocarbon traps, waste disposal control, rock fracture detection in environmental logging, and monitoring of salinity distributions.
The desire to detect such features in the vicinity of boreholes and further away in the formation led to the development of a variety of logging tools. Some of these tools employ sets of antennae for emitting and receiving high frequency electromagnetic waves. By measuring for example the attenuation of the waves, these tools can be used to determine formation properties such as relative permittivity and conductivity and their frequency dependence or dispersion. By fitting the dispersion curves to petrophysical models such as the so-called mixing laws and rock models, it is possible to determine a number of petrophysical parameters related for example to water volume fraction, water DC conductivity, salinity, rock pore and grain system etc.
For the detection of structural features, electromagnetic waves have been used as borehole radar. The electromagnetic wave propagating in the medium can be described by the well-known Maxwell Equations. When the time-related nonlinear effects of the formation, i.e. EM mode conversion and movement related Doppler shift etc., are ignored, the electromagnetic wave propagating and reflected inside the formation can be described by equation [1] below:
                                          ∇            2                    ⁢                                    E              ⇀                        ⁡                          (                                                r                  ⇀                                ,                ω                            )                                      =                                            -                              ω                2                                      ⁢                          μ              0                        ⁢                          μ              r                        ⁢                                          ɛ                0                            ⁡                              (                                                                            ɛ                      r                                        ⁡                                          (                      ω                      )                                                        -                                      ⅈ                    ⁢                                                                  σ                        ⁡                                                  (                          ω                          )                                                                                                                      ɛ                          0                                                ⁢                        ω                                                                                            )                                      ⁢                                          E                ⇀                            ⁡                              (                                                      r                    ⇀                                    ,                  ω                                )                                              -                                    ⅈωμ              0                        ⁢                          μ              r                        ⁢                                                            J                  ⇀                                0                            ⁡                              (                                                      r                    ⇀                                    ,                  ω                                )                                                                        [        1        ]            
This and similar equations, which neglect the possibilities of EM mode conversion and Doppler shift are used or assumed valid in most of the known radar detection and borehole dielectrics spectroscopy logging methods.
In radar-type detection methods, electromagnetic wave pulses are emitted and reflected from structures which show an impedance contrast to the background medium. The pulses are usually tuned to one or more center frequencies and the receiving signal is filtered by a narrow bandpass filter to remove other frequencies. When reflected from a feature in the formation, the arrival time of the reflected wave is measured and evaluated to determine the distance of the feature from the borehole.
A relatively early example of measuring the dielectric constant through phase shift and attenuation measured at each center frequency to determine subsurface formation parameters is described in the U.S. Pat. No. 3,944,910 to Rau. Further developments of this method can be found for example in the U.S. Pat. No. 4,678,997 to Janes and in the U.S. Pat. No. 5,132,623 to De and Nelson. The U.S. Pat. No. 5,168,234 to Freedman describes further variants of this method, as does the U.S. Pat. No. 5,059,907 to Sherman. More recently, the U.S. Pat. Nos. 7,376,514 to Habashy et al. and 7,363,160 to Seleznev et al. disclose more variants of this method.
Radar methods which depend on determining the arrival time of reflected waves are described in many published documents. For example the U.S. Pat. No. 4,814,768 patent to Chang teaches the use of low-frequency radar pulses to detect reflections from geological discontinuities. Further variants of borehole radar systems are described in the U.S. Pat. No. 4,670,717 to Sender, U.S. Pat. No. 4,297,699 to Fowler et al., U.S. Pat. No. 4,430,653 to Coon and Schafers. More recently, further variants of radar based methods are described in U.S. Pat. No. 5,530,359 to Habashy and Xia, U.S. Pat. No. 5,552,786 to Xia et al. and U.S. Pat. No. 6,525,540 to Kong et al.
An airborne ultra-wide band ground penetrating radar (GPR) system providing non-invasive detection and three-dimensional mapping of underground objects and voids is disclosed in U.S. Pat. No. 5,673,050 to Mousally et al.
In view of the known art, it is seen as one object of the invention to improve and enhance single well logging tools and methods. It is seen as a particular object of the invention to provide novel devices and methods to detect boundaries of objects near and further away from the wellbore using broadband electromagnetic wave signals.